In the aftermath of the tragic suicide at Dignitas in Switzerland of Daniel James, the 23-year-old British man paralysed in a rugby accident, Mary Warnock made an elegantly reasoned argument for legalising assisted suicide. But I disagree with her conclusion.

Impassioned arguments for legalisation by other commentators often fail to consider the implications of taking such a radical step. US Supreme Court chief justice Oliver Wendell Holmes wrote that "hard cases make bad law". The cases of Diane Pretty, Debbie Purdy and Daniel James are indeed hard, involving as they do a fully functioning mind trapped in a body that is – or soon will be – an unresponsive shell. It's no coincidence that the assisted suicide cases that have been the subject of high court rulings in the UK, the US and Canada have involved patients suffering from motor neurone disease (called ALS or Lou Gehrig's disease in the US).

In several commentaries I have read, logic has vanished into a terminological fog as disparate terms – assisted dying, assisted suicide, physician-assisted suicide and mercy killing are confused and conflated. Those who disagree are labeled as religious zealots. While it's true that many opponents are motivated by religious convictions, others – such as myself – are not.

The rush to legalisation neglects a fundamental difference between legal and moral judgements. While what is legal and what is ethical often overlap, in compelling cases, there's a strong temptation to set a precedent that can become problematic when enshrined in laws applying to everyone and every case.

I oppose legalisation, because of the unavoidable risk of abuse, a risk that is particularly clear in the American context in which I've spent most of my life. Our radical individualism undermines solidarity. Our medical culture is rife with self-interest and distrust, engendered by extreme litigiousness and insurance companies that stand between patients and their physicians.

In 1997, when the constitutionality of physician assisted suicide came before the US Supreme Court, solicitor general Walter Dellinger, representing the Clinton administration, made an argument with which I agree: terminally ill patients have a liberty interest in not having the state prevent their relief from severe pain and suffering through physician-assisted suicide. However, this interest is trumped by the need to protect vulnerable patients. The systemic dangers are "dramatic" in a society that allows doctor-assisted suicide, Dellinger said. "The least costly treatment for any illness is lethal medication." Although the court found no constitutionally protected right to physician-assisted suicide, individual states could choose to permit it. Eleven years on, only Oregon has made that choice.

Britain has been my second home long enough for me to see the solidarity of NHS fraying in the face of financial pressures. Medical care for the elderly can fall below the standard of adequacy and even common decency. Financial considerations are pervasive. GPs are receiving incentives not to refer patients to hospital. Rationing decisions for cancer drugs are often in the news. Rationing will come to resemble battlefield triage as the population ages and more expensive technologies and drugs become available. In this environment, legalised assisted suicide would allow or even invite coercion. Those who are in pain or receiving sub-standard care are in no position to make a voluntary choice. Going gently into that good night is certainly the least costly alternative.

The law requires bright lines. Could the line hold if Lord Joffe's bill becomes law and physician-assisted suicide is permitted for the terminally ill? One could argue that terminal illness isn't a morally relevant criterion; others who are ill – like Debbie Purdy – suffer grievously. The most acute suffering can be psychological. What about a paralysed young person who has lost the will to live – like Daniel James or Vincent Humbert in France? An elderly person with nothing to look forward to can also suffer – like the healthy 79-year-old German woman who was helped to die? Some years ago, a Dutch jurist named Huib Drion suggested that a suicide pill be made available to all 75 year olds who lived alone. Don't we find ourselves at the bottom of a slippery slope after a slide that was as incremental as it was inevitable?

How should British courts deal with someone who has taken a loved one to Switzerland to die and then returns to the UK? In my view, assisting a suicide requires more than accompanying someone on an airplane. The way forward is not to change the law in Britain to make it consistent with Swiss law, which allows for the actions of Dignitas. Neither the Dutch nor the state of Oregon offers end-of-life services for non-residents. They require the existence of an established doctor-patient relationship through which mental state and the durability of the request can be assessed. The Swiss have sunk to the lowest common moral denominator. The British people deserve better.